The invention relates to a refractory batch for the production of a shaped body and to a process for its production.
Phenolic resin-bonded or pitch-bonded bricks based on magnesia and other oxides as well as graphite are preferably used to line metallurgical vessels. Very high demands are imposed on the performance of the bricks at application temperatures of up to 1800° C. with aggressive, moving slags.
The wear to refractory bricks in use can be roughly divided into two different mechanisms: firstly, the wear caused by chemical reactions (corrosion and oxidation), and secondly thermomechanical wear (cracks, flaking, fatigue of the brick substance). There are also mixed forms, such as abrasion and erosion. While the chemical stability can be influenced in particular by the choice of raw materials (LC sinter, fused magnesia, flake graphite, etc.), the thermomechanical resistance is determined above all by the bonding. In use, MgO-C bricks in principle have four possible ways of compensating for thermomechanical loads; by elastic deformation, by plastic deformation, by microcracks or by macrocracks in the brick structure. While the elastic component of the deformation is naturally low in coarse ceramic products, macrocracks lead to destruction and loss of brick substance.
Under the high application temperatures in the metallurgical vessels, the binders phenolic resin and coal-tar pitch are carbonized to form carbon. The binder is therefore only a means to an end. However, the nature of the resulting carbon, which is responsible for bonding in the bricks under the high application temperatures, is determined by the binder. The nature of phenolic resin bonding means that it has the drawback, compared to pitch bonding, that the carbon which is formed during carbonization (glassy carbon) is rigid and brittle. Pitch-bonded bricks, with high strengths, have relatively low moduli of elasticity. The primary difference is the crystallinity of the carbon, which in pitch results from the formation of a liquid so-called mesophase. Corresponding structures are produced from the phenolic resin under standard conditions only at temperatures of over 2500° C. Unlike crystalline graphite, glassy carbon bonding in practice offers no way of compensating for excess stresses apart from by macrocracks. The result in practice is a higher sensitivity to thermomechanical stresses and mechanical impact loads. Moreover, the isotropic glassy carbon reacts more readily with oxygen, i.e. is more sensitive to oxidation. In use, this may lead to a more rapid loss of brick substance.
The pitch bonding, which is based on coal-tar pitch, however, has the considerable drawback that, when the pitch and the bricks are heated, carcinogenic substances, such as benzo(a)pyrene, may form, and these substances have to be removed from the brick immediately after they have been produced using complex heat treatment methods. Therefore, pitch bonding is under pressure with regard to health and safety at work and environmental protection. The use of newly developed, alternative pitches originating from petroleum generally leads to a reduction in the performance of the bricks. Therefore, there is a need for bonding with optimum use properties, in particular a high flexibility and resistance to oxidation of the bonding coke, in combination with environmentally compatible emissions during production and use.
It is known from “Chemical Abstracts”, Vol. 109, No. 20, Nov. 14, 1988, Abstract No. 1753313e, to add 3-20% by weight of metallic aluminum or aluminum alloy powder and 0.5-7% by weight of chromium oxide powder to resin-bonded magnesia-carbon bricks. This is intended to improve the resistance to oxidation/corrosion.
To accelerate and control the liquid-phase pyrolysis of industrial hydrocarbon mixtures, in particular bonding pitches for refractory shaped bodies, it is known from DE 43 12 396 A1 to add, for example, ferrocene in order to increase the yield of coke. This allows the coke yield to be catalytically increased.